Ultrasonic microphone



5mm MM D FT REFERENCE Aug. 23, 1966 P. HAGEY ULTRASONIC MICROPHONE Filed March 19, 1963 j@ MM,

W I i ma j e lllllllqlll- United States Patent O 3,268,855 ULTRASONIC MICROPHONE Lee P. Hagey, Buchanan, Mich., assignor to Electro-Voice Incorporated, Buchanan, Mich., a corporation of Indiana Filed Mar. 19, 1963, Ser. No. 266,435 9 Claims. (Cl. 340-15) The present invention relates to ultrasonic microphones and more particularly to ultrasonic microphones for actuating control circuits.

The present invention is particularly suited for remote control systems in which an originating station generates an ultrasonic wave which is received by a receiving station and is utilized to actuate an electrical mechanism. Such systems have been used to control the selection of frequency channels in television receivers, control of the volume of such receivers, and turn such receivers on and off".

It is desirable to generate as large an elzctrical signal as possible responsive to the ultrasonic control wave, and hence it is an object of the present invention to provide a microphone for use in a control system utilizing ultrasonic waves which produce a relatively large electrical output, which is extremely reliable, and which is relatively low cost. Piezoelectric transducers have been employed prior t-o the present invention in ultrasonic microphones of this type. However, the electrical output achieved by such piezoelectric microphones has been lower than desired. The present invention is directed toward providing an ultrasonic microphone employing a piezoelectric transducer which achieves greater output than prior equivalent devices.

In the manufacture of ultrasonic microphones prior to the present invention, the cost of such microphones has been greater than desired. The relatively high cost of the prior ultrasonic piezoelectric mircophones has been due to relatively large assembly costs, and also to the necessity of correcting faulty units. It is an object -of the present invention to provide an ultrasonic microphone design utilizing a piezoelectric transducer which may be assembled at relatively small cost and which is highly reliable.

The invention, both as to its organization and principle of operation, together with other objects, features and advantages, will become apparent to those skilled in the art from the following detailed description of an embodiment of the invention, particularly when read in connection with the accompanying drawings, in which:

FIGURE 1 is a front view in elevation of a microphone constructed according to the teachings of the invention;

FIGURE 2 is a sectional view of the microphone taken along the line 2-2 of FIGURE l;

FIGURE 3 is an exploded view of the microphone of FIGURES 1 and 2;

FIGURE 4 is a rear view in elevation of the crystal assembly of the microphone of FIGURES 1 through 3;

FIGURE 5 is a sectional view of the crystal assembly taken along the line 5-5 of FIGURE 4; and

FIGURE 6 is a schematic electrical circuit diagram illustrating a preferred circuit employing the microphone of FIGURES 1 through 5.

In the FIGURES, the microphone has a housing 10 which is generally cup shaped with a cylindrical side wall 12 and a front wall 14. The front wall 14 is provided with a circular central opening 16 designed to face the source of the sound to be received by the microphone. Adjacent to the rear edge of side Wall 12 is an annular, outwardly directed flange 18 extending to an open cylindrical end portion 20 of the housing 10. The housing 10 is constructed of electrically cond-ucting material, such as ICC aluminum or brass. Within the housing 10 adjacent t-o the front wall 14, a circular electrically conducting mesh screen 22 is disposed, the screen 22 being of sufficiently great diameter to lit within and against side wall 12 and front wall 14 of the housing 10. This screen forms an electrically conductiing member covering the opening 16 in a manner which allows sound waves to enter the housing cavity. A small diameter disc 24 is secured centrally on the screen 22 within the opening 16 in the front Wall 14 of the housing 10. This disc 24 serves as a shield at the center of the opening 16 of the housing to prevent sound waves from entering the housing on the axis of the housing, as will be explained hereinafter more fully.

A crystal mounting assembly 26, shown in greater detail in FIGURES 4 and 5, is disposed within the housing 10 in abutment with the screen 22. This crystal -rnounting assembly 26 supports at its center a rectangular piezoelectric plate 28. The plate 28 has a rectangular front face c-overed by an electrially conducting layer or electrode 30 and a rectangular rear face covered by a second electrically conducting layer or electrode 32. The plate 28 is formed by two thin rectangular slabs 29 and 31 of piezoelectric material such as barium titanate, lead titanate, zirconium titanate, or the like; and the slabs 29 and 31 are assembled as a unit with an electrically conducting layer 33 sandwiched between the slabs. The slabs 29 and 31 are polarized electrically in opposite directions relative to the layer 33.

The crystal mounting assembly 26 has a body 34 of electrically insulating material, such as polyethylene plastic, which extends about the plate 28 in virtually the same plane as the plate 28. The body 34 also has a circular outer periphery which is snugly disposed within the side wall of housing 10. The body 34 has an integal cylindrical flange 36 which extends from the periphery of the body 34 away from the front wall 14 and engages the inner surface of the cylindrical side wall 12 to position the crystal mounting assembly 26 within the housing 10.

The body 34 has an X-shaped opening 37 at its center, the X-shaped opening being suflicently greater in size than the rectangular plate side dimensions so that the plate iits inwardly of the body with no interference. Secured to the rear and front faces of the insulating body are metallic contact members 40 and 42 respectively. These contact members 40 and 42 are identical in shape and in general function. Each contact member has a circular outer periphery 43 with a diameter slightly less than the diameter of iiange 36. The members 40 and 42 are secured on opposed sides of the body 34 by four bosses 44 which extend from both sides of the body 34 and exl tend through aperatures 45 in the contact members. The

bosses 44 are heat pressed onto the contact member 40 and 42 to secure the body 34 and contact members 40 and 42 into an assembly. The body 34 also electrically separates and insulates the contact members 40 and 42 from one another.

Each of these contact members 40 and 42 has an X- shaped opening 47 at its center. The opening 47 of each contact member is formed by legs 50, 52, 54, and 56 each disposed on one of two axes at degrees with respect to each other. The tips formed by the leg intersections extend inwardly further than the opening 37 of the body 34, and these tips are numbered 60, 62, 64 and 66. The contact tips 60, 62, 64 and 66 of the members 40 and 42 confront each other in pairs and are provided with bends toward each other to -form four pairs of points disposed at the corners of a square for mounting the piezoelectric plate 28. Each of these four pairs of points engage the opposite surfaces of the plate 28 centrally of one of the sides of the plate so that the plate 28 is mounted adjacent to its edges at four regions located on two orthogonally Irelated central axes of the plate which are perpendicular to the edges of the plate.

Each leg of the X-shaped opening 47 is not wholly rectangular in that the outwardly extending edges numbered 68 and 70 of cut out leg 50 are unequal in length to form a tab 74 at one of the outer corners of the leg. The outer edge 72 of the leg 50 intersects one outward edge 70 and is spaced from intersection with the other edge 68 by the rectangular tab 74. Tab 74 is slit along 76 which is a line continuation of outwardly directed edge 68 to hinge the tab 74 to the contact member 40 along the line 78 which is a continuation of sector wall 72. Tab 74 is angled acutely away from the piezoelectric plate 28 to contact an adjacent conducting member for completion of an electrical circuit. At one outer corner of each of the other legs 52, 54 and 56 there is formed a similar tab 74 so that there is a set of four spaced angularly disposed tabs 74 extending from each contact member 40 and 42, set 80 extending rearwardly from rear contact member 40 and set 82 extending forwardly from contact member 42. It was noted previously that contact members 40 and 42 are identical in size and shape hence what has been described for member 40 holds equally for member 42.

With the construction set out, each plate face 30 and 32 is conductively connected to the adjacent contact member 42 and 40, respectively, by the inwardly angled member tips 60, 62, 64 and 66, each physically contacting the plate adjacent its outer edges and centrally of each edge, thereby leaving the plate center free to vibrate in a direction normal to the surfaces of the plate in response to received sound waves. From each contact member 40 and 42 there extends a tab set 80 or 82 for completing an electrically conducting outward path from the plate 28.

Within the flange 36 rearwardly of the plate 28, there is an electrically conducting flat circular disc 86. Disc 86 has a diameter slightly smaller than the inner diameter of the flange 36 to fit within flange 36 with little circumferential clearance but with suicient clearance to allow the disc to be freely fitted into the flange. This disc 86 is spaced from the adjacent plate face 32 by the rearwardly extending tab set 80. The disc 86 is compressed into physical contact with the tab set 80 by a conical helical spring 88. The base or larger dimension of the conical spring 88 abuts disc 86 to hold the disc firmly against the contact member tab set 80. The end of spring 88 opposite the disc 86 is configured cylindrically and is disposed about the central terminal 89 of a connector 90 and abuts a shoudler 91 of the central terminal 89. The central terminal 89 of the connector extends through an insulating disc base 94 and is disposed within and insulated from a cylindrical shell 92 of electrically conducting material, this portion of the inner conductor 89 being in the form of a hollow sleeve 95 for receiving a pin type connector. The disc base 94 tits within the side wall of housing 10 and serves to compress the conical spring 88 axially against the disc 86. A blade terminal 96 extends from the shell 92 adjacent to the surface of the disc base 94 and through a slot 97 in the disc base 94 to abut the inner surface of the side wall 12 to form an electrical connection from the shell 92 to the side wall of the housing 10. Thus, blade 96 is in physical contact with the inner surface of the housing 10.

To complete the assembly, there is provided an insulating washer 98 which fits within the stepped flange 18 of side wall 12 in abutment with the surface of the disc 94. The washer is circular and includes a circular opening for the cylindrical shell 92 of the connector 90. The rearmost end of the housing side wall is crimped at 99 to hold the assembly firmly in place within the housing as described.

It will be noted that electrical contact between the two faces 30 and 32 of the piezoelectric plate 28 and the two elements of the connector formed by the sleeve 9S and shell 92 is accomplished without any soldering connections, or the like. It is only necessary to assemble parts in the housing and electrical contact is achieved. Further, it is only necessary to crimp the perimeter of the housing 10, as at 99, to retain the assembly in assembled condition, and no screws, bolts, or other operations are required. In practice the screen 22 is first inserted into the casing 10, the disc 24 having previously been positioned on the screen. Thereafter, the crystal assembly 26, the piezoelectric plate 28 having previously been mounted in position, is slipped into the housing 10 with the flange 36 extending rearwardly from the screen 22. The electrically conducting plate 86 is then placed in abutment with the tab set 80, and the connector with attached spring 88 is slipped into position. The washer 98 is then slipped into abutment with the disc 94 of the connector 90, and the crimps 99 are made in the exterior surface of the flange portion 18 of the housing 10. By merely inserting the connector 90 into the housing 10, the leg 96 is placed into abutment and good electrical Contact with the electrically conducting housing 10 to provide electrical contact to the electrode 30 of the ceramic piezoelectric plate 28. In like manner, the spring 88 and electrically conducting disc 86 provide electrical contact to the electrode 32 through the tab set 80 of the contact member 40.

A cavity 100 is formed between the disc 86 and the piezoelectric plate 28 by means of the crystal assembly 26. The tab set 80 holds the disc 86 at a relatively fixed distance from the confronting surface or electrode 32 of the piezoelectric plate 28, theerby forming the cavity. It is to be noted that the piezoelectric plate 28 is held in relatively fixed position at four points adjacent to its perimeter. These points being located centrally of the four edges of the plate 28, are located approximately on the nodal circle or figure of zero displacement for the fundamental mode of resonance of the piezoelectric plate 28. As a result, the center and corners of the plate move relative to the plane of the four points in response to an impinging sound wave. The disc 24 prevents an impinging sound wave from directly striking the center of the crystal plate 28 in order to utilize a larger area opening 16 without permitting the short path directly to the center of the plate 32 to excite the plate 32 at a different phase angle than the sound waves entering through the peripheral portions of the opening 16.

The cavity 100 is designed to resonate at the frequency of the impinging sound waves. The mechanical resonance formed by the cavity 100 at this frequency increases the electrical output of the piezoelectric plate by approximately 4 decibels for impinging sound waves having a frequency of approximately 25 kilocycles per second. In one particular construction designed for operation at this frequency, the housing 10 has an inner diameter of approximately l inch, and the inner diameter of the flange 36 is about 0.8 inch. The tab set 80 positions the disc 86 from the confronting surface of the piezoelectric plate 28 by a distance of approximately 0.02 inch, which produces the desired resonance. The piezoelectric plate 28 is in the form of a square having dimensions of approximately 2%; inch on each side. The legs 50, 52, 54 and 56 have dimensions normal to their axes of approximately %2 inch. The disc 86 is constructed of aluminum, as is the casing 10 and the screen 22. The shield 24 is constructed of polyethylene plastic, and the body 34 of the mounting assembly 26 is also constructed of polyethylene plastic.

FIGURE 6 tillustnates the electrical circuit used with the microphone tillustnatted in FIGURES 1 through 5. The piezoelectric genenatiimg element 28 .is illustrated Wlilth its equivalent electrical circuit symbols, namely a generator 102, capacitors -104 and 120, and inductance 106. The plate 28 ii-s lillustnated electrically connected to Ian amplifier which includes la vacuum (tube 112. The guild 114 of the vacuum tube 112 'alud the cathode 116 lof fthe vacuum :tube 112 :are connecte-d to rbhe electrodes of the piezoeleetlnic platte t28. The amplifier 110 also has a maniable inductor 11|8 connected lbetween the gnild 1.14 ,and the cathode 116 thereof, and this inductor 118 resonates with the capacitor 120 of 'the piezoelectric -pla1te 28 at the frequency of the impinging .sound wave. lIn this manner, the piezoelectric plate is closely coupled to .the input off the amplilier d10, thus resulting in a very broad frequency to output characteristic for the microphone. Hence, the microphone is less critical as to impinging frequencies than it would be were this resonant circuit employing the inductor 118 omitted.

Those skilled in the ant will readily see that a microphone has been provided which is extremely simple in construction :and assembly, and low cost compared to prior devices. lFurther, .fthe microphone set forth hereinbefore produces lan improved electrical output over prior rant devices, :and further provides 'an improved electrical output over -a relatively wide range of input frequencies considering ultrasonic control microphones as a cla-ss. Those skilled in the ,ant will find many applications for the microphone set forth herein in addition to those described, and will also find modifications to the microphone hereinbefore described within the intended scope of the present invention. It is therefore intended that the scope of the present invention be not limited by the foregoing disclosure, but rather only by the appended claims.

The invention claimed is:

1. A microphone compri-sing, in combination, an electrically conducting housing having lan opening in the front tace thereof for receiving impinging sound Waves, an electrically conducting screen disposed within the housing confronting said opening land abutting the housing to form electrical conta-ct with said housing, la piezoelectric plate carrying :a pair of electrodes on opposed faces thereof, a first means disposed between the screen land plate having a plurality of electrically conducting tips in contact with one of said electrodes at a plurality of points on the nodal circle adjacent the edges of the plate, sai-d contacting means including la plurality of tabs of electrically conducting matenial interconnected with said tips and engaging said screen to electrically interconnect said housing and one electrode on said plate, 'a second means disposed adjacent t-o the other electrode of said plate having a plurality of conducting tips in contact with said other electrode rat a plurality of, points disposed on the opposite side of the plate from the points of contact of the first means, la plurality of tabs of electrically conducting m terial electrically connected to the tips of said second means rand extending from said second contact means, an imperforate electrically conducting `disc abutting said last mentioned tabs, said last mentioned tabs spacing said disc a predetermined distance from the other face to provide an air cavity on .the side of the plate remote from said housing opening.

2. A microphone comprising the elements of claim 1 in combination 'with la mass of material smaller than the opening in the housing disposed on the screen centrally of the opening.

3. An ultrasonic microphone comprising :a quadnangular .piezoelectric plate having a pair of opposed square faces provided with electrodes thereon, a mounting structure for said plate including la first member of electrically insulating material having a hat portion and lan opening located centrally of the flat portion, the piezoelectric plate being disposed Within said opening and lying in the plane of the fiat portion of said -first member, said first member being provided with a yflange 'wall extending trom the perimeter of the flat portion of the member, said mounting structure having a lsecond member of electrically conducting material mounted on one side of the fiat portion of the first member and having four spaced tips extending therefrom, each tip terminating in engagement with the same electrode of the piezoelectric plate [adjacent to the perimeter of said piezoelectric plate land centrally of one o-f the edges thereof, said mounting structure having a second member of electrically conducting material mounted on the other side of the flat portion of the insulating member and having four spaced trips extending therefrom, each tip engaging the same electrode of the piezoelectric plate on the side of said plate opposite the points of conftact of the tips of the second member, 'whereby the piezoelectric plate is mounted in fixed position relative to the mounting structure along kfour axes normal to the electrodes thereof and disposed centrally of the tour edges thereof, said second contact member having la plurality of protruding tabs extending therefrom, a rdisc disposed 'within the flange Wall of the first member of the mounting structure in .abutment with the tabs of the second mem- Iber, said ldisc being of electrically conducting material and forming with the piezoelectric plate and mounting structure Ia cavity having a resonant frequency within the responsive range of frequencies of [the microphone, a housing disposed about the mounting structure having an opening conrontin g the second member Iof the mounting structure and the piezoelectric plate, and electrical terminal means ymounted on the housing having a first terminal electrically connected to the second member of the mounting structure and a second terminal electrically connected to the third member of the mounting structure.

4. An ultrasonic microphone comprising the elements of claim 3 wherein the housing has lan aperture therein confronting the disc and the terminal means is mounted cn said housing and extends through the aperture thereof, said terminal means including a spring mounted at one end on the termi-nal means and terminating lat the other end in abutment wit-h the disc, said lspring maintaining the disc in abutment with the tabs.

5. An ultrasonic microphone comprising, in combination, a Cup-shaped housing having a cylindrical portion extending from a flat face, the fiat face having a circular opening therein, said housing having a second cylindrical portion of larger diameter than the first cylindrical portion mounted on the end of the first cylindrical portion opposite the flat face and terminating in a circular orifice, said housing being of electrically conducting material, an electrically conducting circular screen disposed within the housing in abutment with the flat face thereof and extending over' .the opening, said screen having a circular mass of material having a diameter smaller than the opening disposed centrally of the opening, a quadrangular piezoelectric plate having a pair of opposed square surfaces, an electrically conducting layer forming an electrode disposed on each of the fiat surfaces, and means disposed between the flat surfaces of the piezoelectric plate forming a third electrode, said piezoelectric plate being electrically polarized between said third electrode and the two other electrodes in opposite directions relative to said third electrode, a mounting structure for the piezoelectric plate including a first member of electrically insulating material having a flat circular portion with a diameter approximately equal to the interior diameter of the first cylindrical portion of the housing and a cylindrical flange extending from the periphery of the circular flat portion of the first member in a direction opposite the face of the housing, said flat portion of the first member having an aperture centrally therein for accommodating the piezoelectric plate, said aperture being formed with four leg portions disposed on two orthogonal axes intersecting at the center of the flat portion, the first and third leg portions being disposed on the same axis and the second and fourth leg portions being disposed on the same axis and having a combined length approximately equal to the combined length of the first and third leg portions, said mounting structure having a second member of electrically conducting material mounted ion the 4first member n the side thereof confronting the fiat end of `the housing, said second member having an aperture centrally thereof smaller than the aperture of the first member, said aperture of the second member having four leg portions disposed on two orthogonally related axes crossing at the center of the second member forming an X-shaped aperture, the junction of the first and second leg portions of said aperture forming a pointed tip, the junction of the second and third leg portions forming a second pointed tip, the junction of the third and fourth leg portions forming a third pointed tip, and the junctions of the fourth and first leg portions forming a fourth pointed tip, said pointed tips being bent into the aperture of the first member of the mounting structure, and said mounting structure having a third electrically conducting member mounted on the surface of the first member opposite the second member, said third member having a central aperture with four leg portions disposed on two orthogonally related axes intersecting at the center of said aperture forming an X-shaped aperture, the aperture of the third member being aligned with the aperture of the second member and the aperture of the first member, the junction of the first and second leg portions of the aperture of the third member forming a first tip, the junction of the second and third leg portions of the aperture of the third member forming a second tip, the junction of the third and fourth leg portions of the aperture of the third member forming a third tip, and the junction of the fourth and first leg portions of the aperture of the third member forming a fourth tip, the tips of said third member extending intio the aperture of the first member and directly confronting the tips of the second member, the piezoelectric plate being disposed in the aperture of the first member with the tips of the second and third member abutting the electrodes of the piezoelectric plate on opposite surfaces of the piezoelectric plate and centrally of the edges of the piezoelectric plate, said second member having a tab protruding therefrom adjacent to one edge of each of the leg portions, said tabs extending away from the first member and abutting the screen, said third member having a tab extending therefrom adjacent to each of the leg portions thereof, said tabs extending away from the first member, an electrically conducting imperforate disc having a diameter approximately equal to the inner diameter of the cylindrical ange of the first member disposed within the fiange and abutting the tabs of the third member, and la contact assembly having a disc of electrically insulating material disposed Within the second cylindrical portion yof the housing and secured in position, said contact assembly having a central member extending through a central aperture of the insulating disc and a blade portion extending about the periphery of the insulating disc into abutment with the first cylindrical portion of the housing, and a cone-shaped spiral spring having one end in abutment with the electrically conducting disc and the other end engaging the central member `of the contact assembly.

6. A microphone comprising the elements of claim in combination with an inductance connected between the blade portion and central member of the contact assembly, said inductance beingvresonant with the piezoelectric plate at a frequency within the responsive range of the microphone, whereby sound Waves impinging upon the piezoelectric plate having a frequency approximately equal to said frequency of resonance set the piezoelectric plate into vibration.

7. An ultrasonic microphone comprising: a quadrangular piezoelectric plate having a pair of opposed square faces provided with electrodes thereon, a contact structure for said plate including a member of electrically insulating material having a fiat portion and an opening therein disposed about the plate, said member being in the same plane as the plate and being provided with a flange wall extending from the perimeter of the fiat portion of the member, a first contact member of electrically conducting material mounted on one side of the fiat portion of the insulating member having four spaced tips extending therefrom, each tip engaging the same electrode adjacent the perimeter thereof and centrally of one `of the edges thereof, a second contact member mounted on the opposite side of the fiat portion of the contact structures electrically contacting the other of the electrodes of said plate adjacent the edges thereof, disc means fitted within said flange wall for completing a conducting path from said second contacting member to an external circuit, means spacing said disc means from said other electrode whereby to form an air cavity between said plate and said disc, a housing fitted about said annular ange, an opening in one end of said housing in communication with said one face of said plate, and means completing a conducting path from said first contacting member to said housing while maintaining a path of communication between said one electrode and said housing opening.

8. A microphone comprising, in combination, a quadrangular piezoelectric plate having a pair of opposed square faces provided with electrodes thereon, and means for mounting said plate comprising four pairs of opposed tips, each of said pairs of tips being disposed in the same plane, and each pair of tips being located centrally of one of the edges of the piezoelectric plate, each tip of each pair engaging one of the faces of the piezoelectric plate adjacent to the edge of said plate.

9. A microphone comprising the combination of claim 8 in combination with an imperforate body having a cross sectional area smaller than the piezoelectric plate mounted confronting the central portion of the one face of the piezoelectric plate and spaced from said piezoelectric plate.

References Cited by the Examiner UNITED STATES PATENTS 975,596 11/1910 Young 179--181 2,488,290 11/1949 Hansell 310-8.7 3,007,013 10/1961 Paull et al 179--1 10.1 3,109,111 10/1963 Wiggins 310-8.7

FOREIGN PATENTS 409,040 4/ 1934 Great Britain.

BENJAMIN A. BORCHELT, Primary Examiner. P. A. SHANLEY, Assistant Examiner. 

3. AN ULTRASONIC MICROPHONE COMPRISING A QUANDRANGULAR PIEZOELECTRIC PLATE HAVING A PAIR OF OPPOSED SQUARE FACES PROVIDED WITH ELECTRODES THEREON, A MOUNTING STRUCTURE FOR SAID PLATE INCLUDING A FIRST MEMBER OF ELECTRICALLY INSULATING MATERIAL HAVING A FLAT PORTION AND AN OPENING LOCATED CENTRALLY OF THE FLAT PORTION, THE PIEZOELECTRIC PLATE BEING DISPOSED WITHIN SAID FIRST MEMBER, SAID FIRST MEMBER OF THE FLAT PORTION OF SAID FIRST MEMBER, SAID FIRST MEMBER BEING PROVIDED WITH A FLANGE WALL EXTENDING FROM THE PERIMETER OF THE FLAT PORTION OF THE MEMBER, SAID MOUNTING STRUCTURE HAVING A SECOND MEMBER OF ELECTRICALLY CONDUCTING MATERIAL MOUNTED ON ONE SIDE OF THE FLAT PORTION OF THE FIRST MEMBER AND HAVING FOUR SPACED TIPS EXTENDING THEREFROM, EACH TIP TERMINATING IN ENGAGEMENT WITH THE SAME ELECTRODE OF THE PIEZOELECTRIC PLATE ADJACENT TO THE PERIMETER OF SAID PIEZOELECTRIC PLATE ADJACENT TO THE OF THE EDGES THEREOF, SAID MOUNTING STRUCTURE HAVING A SECOND MEMBER OF ELECTRICALLY CONDUCTING MATERIAL MOUNTED ON THE OTHER SIDE OF THE FLAT PORTION OF THE INSULATING MEMBER AND HAVING FOUR SPACED TIPS EXTENDING THEREFROM, EACH TIP ENGAGING THE SAME ELECTRODE OF THE PIEZOELECTRIC PLATE ON THE SIDE OF SAID PLATE OPPOSITE THE POINTS OF CONTACT OF THE TIPS OF THE SECOND MEMBER, WHEREBY THE PIEZOELECTRIC PLATE IS MOUNTED IN FIXED POSITION TO THE ELECMOUNTING STRUCTURE ALONG FOUR AXES NORMAL TO THE ELECTRODES THEREOF AND DISPOSED CENTRALLY OF THE FOUR EDGES THEREOF, SAID SECOND CONTACT MEMBER HAVING A PLURALITY OF PROTRUDING TABS EXTENDING THEREFROM, A DISC DISPOSED WITHIN THE FLANGE WALL OF THE FIRST MEMBER OF THE MOUNTING STRUCTURE IN ABUTMENT WALL OF THE TABS OF THE SECOND MEMBER, SAID DISC BEING OF ELECTRICALLY CONDUCTING MATERIAL AND FORMING WITH THE PIEZOELECTRIC PLAT EAND MOUNTING STRUCTURE A CAVITY HAVING A RESONANT FREQUENCY WITHIN THE RESPONSIVE RANGE OF FREQUENCIES OF THE MICROPHONE, A HOUSING DISPOSED ABOUT THE MOUNTING STRUCTURE HAVING AN OPENING CONFRONTING THE SECOND MEMBER OF THE MOUNTING STRUCTURE AND THE PIEZOELECTRIC PLATE, AND ELECTRICAL TERMINAL MEANS MOUNTED ON THE HOUSING HAVING A FIRST TERMINAL ELECTRICALLY CONNECTED TO THE SECOND MEMBER OF THE MOUNTING STRUCTURE AND A SECOND TERMINAL ELECTRICALLY CONNECTED TO THE THIRD MEMBER OF THE MOUNTING STRUCTURE. 